Adjustable dock boards or dock levelers are adapted for installation on loading docks to span the gap between the dock and the bed of a truck or other carrier, which is in position to be loaded or unloaded. These devices have been widely used for some time and are well established in the art. Representative of patent literature which describes such devices are U.S. Patents:
U.S. Pat. No. 3,137,017--Pfleger et al: PA0 U.S. Pat. No. 3,299,456--Dieter et al; PA0 U.S. Pat. No. 3,368,229--Pfleger; PA0 U.S. Pat. No. 3,530,488--Beckwitch; PA0 U.S. Pat. No. 3,835,497--Smith; PA0 U.S. Pat. No. 3,858,264--Kuns et al; PA0 U.S. Pat. No. 3,877,102--Artzberger; PA0 U.S. Pat. No. 3,995,342--Wiener; PA0 U.S. Pat. No. 4,014,059--Artzberger et al; PA0 U.S. Pat. No. 4,126,909--Smith et al; PA0 U.S. Pat. No. 4,279,050--Abbott; PA0 U.S. Pat. No. 4,328,602--Bennett; PA0 U.S. Pat. No. 4,455,703--Fromme et al; PA0 U.S. Pat. No. 4,619,008--Kovich et al; PA0 U.S. Pat. No. 4,776,052--Delgato et al.
This type of device generally comprises a ramp or deck which is hingedly mounted at its rear edge to the dock. This connection can be in the form of a hinge directly to the dock or, the entire unit can be mounted in a frame, which is then set into a pit in the dock. The front edge of the deck generally has an extension lip which is hinged thereto to rest on the bed of the truck. As the truck is loaded or unloaded it tends to move vertically or "float", and the dock leveler is responsive to such movement by relative flexing between the lip and the deck.
Another type of vertically storing leveler acts as a bridge between carriers such as railroad cars. In those cases the leveler is mounted on a carriage to move laterally on a track so that the leveler can be aligned with a rail car door. It has a lip. A variation is a vertically storing lift bridge. This is pit mounted, i.e. fixed laterally, and is used to span two fixed abutments. No lip is necessary. In the application the term "bridge" will be applied to both dock levelers and bridges.
Typically, such levelers can be powered hydraulically, mechanically, electrically or the like. They are generally lowered by gravity against damping elements and then raised utilizing a power source of the type just mentioned. One of the problems associated with that mode of operation is that the rotational velocity of the leveler is not uniform and that, at the end of the movements stroke the device can violently rock or shake. Also, as a function of the geometry of the system the force necessary to raise, or lower, if done mechanically, will vary. This can be detrimental on the power system and may be a failure mode in the operation of the device.